Patent Document 1 below discloses a vehicle seat provided with a side airbag device that inflates and deploys a side airbag between a door trim of a side door and the side of a seated occupant in event of a side-on collision. The side airbag device is generally inbuilt into a side portion on a seat width direction outer side of a vehicle seat.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2000-313302